The present invention relates to a dissimilar material weld joint formed by joining an iron type material and an aluminum type material and a weld joining method, which dissimilar material weld joint is suitably applied to transport vehicles such as automobiles and railway vehicles, machine parts, structural members such as building structures, and the like and is particularly needed when structures for automobiles are fabricated.
If members comprising different kinds of metals (dissimilar material joints) of an iron type material (hereunder referred to simply as “a steel material”) and an aluminum type material (a generic name of pure aluminum and aluminum alloys, and hereunder referred to simply as “an aluminum material”) can be joined, it remarkably contributes to weight reduction and the like. However, when a steel material and an aluminum material are weld joined, brittle intermetallic compounds are likely to form at the weld and therefore it has been very difficult to obtain a reliable weld having a high strength (joint strength). Joining with bolts, rivets or the like has heretofore been applied to the joining of dissimilar materials (members of different metals) accordingly, but such joints have problems in reliability, airtightness, cost and others.
In this light, many studies have heretofore been done with regard to the joining method of a dissimilar material joint. Then, the following joining methods have been proposed for example;
a method of joining dissimilar materials by vacuum roll cladding (refer to Patent Document 1),
a method of seam-welding dissimilar materials while interposing a two-layered clad material comprising an iron type material layer and an aluminum alloy layer which have been prepared beforehand (refer to Patent Document 2),
a method of joining dissimilar materials by applying pressure bonding at a high temperature (refer to Patent Document 3),
a method of joining dissimilar materials by interposing beforehand a Ti alloy on the joint surface thereof and then applying HIP treatment (refer to Patent Documents 4 and 5),
a method of joining dissimilar materials by friction welding (refer to Patent Document 6),
a method of joining dissimilar materials by plating a steel material on the surface facing aluminum with aluminum alloy or interposing a two-layered clad metal comprising an steel layer and an aluminum alloy layer prepared in advance, and then applying resistance welding (refer to Patent Documents 7 and 8), and
a method of joining dissimilar materials by arc-welding a steel material and an aluminum material (refer to Non-patent Documents 1 and 2).
The above prior arts however have the following problems.
The methods of forming a dissimilar material joint by joining a steel material and an aluminum material as described in Patent Documents 1 to 8 are generally: applicable to the materials having relatively simple shapes, such as flat plates; but not applicable to members having complicated shapes because of the restrictions from the geometry of the members. As a consequence, the methods are only applicable to limited applications and thus inferior in versatility. Further, another problem is that, by the methods, since a weld is formed spotwise, a continuously extending weld cannot be obtained. Furthermore, another problem is that, since the methods require complicated processes, the stability of quality cannot be secured, the cost incurred in joining increases, and thus the methods lack in practicability. In addition, yet another problem is that the methods cannot be applied to an existing welding line, therefore new equipment has to be introduced if one of the methods is to be applied, and resultantly the equipment cost increases.
In contrast, in the case of the method of joining a steel material and an aluminum material by arc welding as described in the aforementioned Non-patent Documents 1 and 2, a high joint strength can be secured by controlling the direction in which intermetallic compounds acting as an impediment to the securement of strength develop. The method is the one wherein a hole is formed beforehand on the side of a steel material to be joined, then the hole is filled with molten aluminum material, and thereby the steel material and an aluminum material are joined.
One of the basic reasons why such various methods as described above are proposed in the case of the joining of a steel material and an aluminum material is that, when the steel material and the aluminum material melt and are directly joined to each other, brittle intermetallic compounds form at the weld and thus cracking is likely to occur. In that sense, when a steel material and an aluminum material are directly joined including the case of joining them with a welding wire, it becomes extremely important to adopt any one of the following means; 1) how to secure the ductility of the molten metal portion by inhibiting to the utmost iron in the steel material and aluminum in the aluminum material from melting and mixing, 2) how to avoid forming a brittle intermetallic compound layer in the vicinity of the interface between the steel material and the aluminum material, or 3) how to design the shape of a joint that does not hinder the mechanical properties thereof even if brittle intermetallic compounds are formed at the joint interface between the steel material and the aluminum material.
Then from that point of view, the present inventors have variously studied with the aim of establishing a joining method capable of securing a sound weld joint while resolving the problems described in the above items 1) and 2) as much as possible. As a result, the present inventors have found that, when the method is adopted of directly joining a steel material and an aluminum material by the MIG brazing method, a highly reliable weld joint can be obtained while avoiding various problems pointed out in the above prior arts (refer to Patent Document 9).    [Patent Document 1] JP-A No. 94162/2000    [Patent Document 2] JP-A No. 197846/1999    [Patent Document 3] JP-A No. 185040/1998    [Patent Document 4] JP-A No. 198458/1994    [Patent Document 5] JP-A No. 8056/1993    [Patent Document 6] JP-A No. 141755/1996    [Patent Document 7] JP-A No. 39558/1994    [Patent Document 8] JP-A No. 63762/1994    [Patent Document 9] JP-A No. 33865/2003    [Non-patent Document 1] WELDING JOURNAL, (1963), p. 302    [Non-patent Document 2] Light Metal Welding: Vol. 16 (1978), No. 12, p. 8